The Z4 sDrive30i sports a 255-horsepower, 3.0-liter inline-six; a 300-horsepower, twin-turbo version of that engine comes in the sDrive35i. And the difference between these two models, actually, goes far beyond their 45-hp difference in engine output. In short, the sDrive30i drives more like a classic roadster, while the sDrive35i is faster but is best enjoyed in a different way.